The Applicant's published French Patent application No. 2 414 988 described a method of manufacturing a ceramic unit for indirect heat exchange, the method comprising extruding a raw ceramic material in a parallel duct configuration, piercing the ends of at least a first series of the ducts corresponding to one of the indirect heat exchange fluids with inlet and outlet orifices for a fluid to pass through said series of ducts, the axes of the orifices being perpendicular to the common direction of said ducts, and firing the unit to give it the required mechanical strength.
In such a method, the inlet and outlet orifices for one of the fluids are formed by piercing the walls of the ducts at the end of one of the series of ducts. These orifices must be formed not only in the outer walls of the unit, but also in its inner walls which separate adjacent ducts. This gives rise to manufacturing difficulties, since the walls must not be deformed thereby and it is difficult and expensive to form the orifices in the unit after firing. The orifices also cause appreciable loss of head in the finished unit when in use.